1. Field of the Invention
The present invention relates to a method of producing a crystalline material using an automated vision system as part of a crystal growth apparatus.
2. Description of the Related Art
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce a silicon ingot by the DSS method, solid silicon feedstock is provided in a crucible, often contained in a graphite crucible box, and placed into the hot zone of a DSS furnace. The feedstock is then heated to form a liquid feedstock melt, and the furnace temperature, which is well above the silicon melting temperature of 1412° C., is maintained for several hours to ensure complete melting. Once fully melted, heat is removed from the melted feedstock, often by applying a temperature gradient in the hot zone, in order to directionally solidify the melt and form a silicon ingot. By controlling how the melt solidifies, an ingot having greater purity than the starting feedstock material can be achieved, which can then be used in a variety of high end applications, such as in the semiconductor and photovoltaic industries.
In such a method, it is often a challenge to accurately identify when the feedstock has fully melted and/or when the growth of the ingot is complete. Missing either or both of these events in the process can have deleterious effects on the quality of the resulting crystalline material. For example, if a silicon feedstock is fully melted but remains at the high melting temperature for an excessive amount of time, the quantity of contaminants such as carbon and oxygen in the melt can increase, producing impurities that can affect the overall performance of the final silicon ingot. In addition, missing the end of melt also has a substantial negative impact on the rest of the solidification process, particularly on the timing and temperatures of subsequent steps. Furthermore, for the solidification step, improper identification of the completion of growth results in a fully grown solid ingot being subjected to significant thermal gradients, which can cause damage to the final ingot. Typically, the end of melt and the completion of growth are determined based on internal temperature readings and confirmed manually by an operator looking into the furnace. However, this has often proven to be unreliable or, at best, inconsistent.
Thus, there is a need in the industry for methods and devices that can monitor the melting of a feedstock in a crystal growth apparatus in order to determine when melting is complete and/or can monitor the growth of a crystalline material from a fully melted feedstock to determine when growth is complete.